Earlier quoted context omitted.
>They can decide to sell 10 copies, and dictate that if they are resold, the photographer gets a 20% royalty. Suppose that I want to sell my copy, but do not want to pay the 20% royalty. What stops me from selling it for 1 cent on-chain, but buyer paying me real price off-chain? How can a creator realistically expect to receive their royalty cut?
It's an interesting loop hole. That can actually be solved with a smart contract that wouldn't allow you to sell below the purchase price, or at least not that far below it. Anything can be programmed in a smart contract, and they're transparent, so if the royalty rules are not acceptable to you, don't buy it.
I think this is more than just a loophole - it's a fundamental weakness of the smart contract system. The smart contract can audit things which are on the blockchain, but cannot govern external actions. This is fine when the only objects the contract cares about are digital tokens, but once you integrate things like real-world art and associated rights, it becomes very easy to circumvent.